Process for the treatment of alloys of iron



y 19351 K. H. s. LGFQUIST I 2,047,479

PROCESS FOR THE TREATMENT OF ALLOYS OF IRON Filed April 50, 1934 OR: WITH (Fe 0-) oxross SL195 T FUHHHCE SULPH/DE c 1 51-56 HALO) 50 60s Cr-S-O-SLBG- ED857706- F (IR 719C E SECOND Rofis TED REF/M116 PRoouL' T UR/1 965 IMPURE STEEL ELEGTR/C FUR/75c:

llZr/ He] SIJJHaL Lof7uJsf' Patented July 14, 1936 raocn'ss roa THE TREATMENT or armors or mon Karl lfilelge Sigfrid Liiiquist, Stockholm, Sweden Application April 30, 1934, Serial No. 723,273

21 Claims.

In earlier methods for treating such alloys in order to concentrate chromium the following approximate manner of proceeding has been used.

' The molten alloy is subjected to. an oxidation process, for instance one of the ordinary metalvlurgical refining processes. The chromium, which has a higher affinity for oxygen than has iron thus at first, becomes oxidized and slag'ged, while iron, as well as other metals or substances of less aflinity for oxygen, occur as oxides only to a lesser degree and in the major part remain in the metal meltf Carrying out this process requires a fluid slag. Because of the high melting point of the chromium oxide,'the slag obtained becomes viscous when its chromium content increases and the realization of the process is difilcult .to carry out and a poor yield of chromium.

results-. Because of these, circumstances, the use of this method has been very restricted.

The present invention consistsin a process of treating such alloys containing iron andchromium, comprising smelting the alloys with oxygenand sulphur-containing media which slag ofif the chromium of the alloy partly as chromium oxide and partly as chromium sulphide, there being formed a slag which is rich in chromium and which has a relatively low melting point.

mium is oxidized to such a point that in addition to practically the entire chromium content being oxidized, a certain amount of the iron is also oxidized. A slag of low melting point is produced, due to the presence of the relatively fusible oxide of iron, but "its content of chromium will be rather low. This slag, diluted with respect to chromium, is therefore subsequently .reduced with a new metal melt containing chromium. Chromium of this melt will then go over into the slag and replace the iron, as well as other metals or substances in the slag-reducible by chromium, which metals pass over into the metal melt. the slag which would ensue if this concentration in the slagof oxide of chromium is counter- The raising of the melting point of In France May 1, 1933 acted by adding sulphur in the form of elementary sulphur, iron pyrites, pyrrhotite or some other sulphide or sulphurous compound, where sulphur is less strongly chemically combined than in chromium-sulphide.

Becauseof the action of the above-mentioned sulphurous material on the molten iron-chr0- mium alloy part of the chromium in the alloy combines with the added sulphur, formingchromium sulphide, which passesinto the slag. In addition, and as described above, the slag itself contains chromium oxide. When the oxidizing and sulphurizing agents are thus chosen so that they are'capable of being chemically reduced by chromium after the reaction of the sulphurous material with the alloy the slag consists principally of a mixture of oxide and sulphide. of chromium, which mixture is relatively fusible. This is shown in the flow sheet formingpart of -this application. As a matter of fact, in several cases the oxide and the sulphide of a metal are quite miscible in the molten state and the melt has a considerably lower freezing point than that of either of the two components oxide and sulphide. By proper choice of the proportion between oxide and sulphide a minimum of freezingor melting-temperature may be obtained corresponding to that of the eutectic mixture.

For example, while ferrous sulphide, FeS, melts at 1193 C., and ferrous oxide, FeO, at 1370 0., nevertheless a mixture consisting of 42% by weight FeO and 58% FeS melts into a homogeneous slag at only 940 C. and similar conditions have been observed to prevail in the case of mixtures of the sulphide and oxide of chromium,

' and also of nickel.

The amount of chromium which passes into the slag during the reduction is in the first place determined by the amount of oxygen and sulphur available for uniting with the. chromium, or in other words by the original content in the slag of reducible oxide, especially of iron, and of sulphurous compound, capable of sulphurizing chromium. i Y

When chromium passes into the slag in the manner described above, a corresponding amount of iron, as wellas other metals or' substances reducible by chromium, and also the metal which is present in any added sulphide, will be expelled from the slag and will pass into the metal melt. A certain equilibrium eventually will be reached, as a result of which'a certainamount of the chromium remains in the metal melt, in equilibrium with a, certain amount of chromium which has been combined with oxygen and with sulphur 55 in the slag. The more. chromium present in the metal melt after equilibrium is reached, the higher will be the concentration thereof in the slag. The fusible slag, obtained in this way, is poured ofi and treated in a manner which will be described below for extracting the chromium.

As an alternative procedure, the sulphur or the sulphurous compound may have been previously added during the initial refining of the alloy, in order to obtain a. low freezing point of the slag, due to the presence of sulphide, mainly chromium sulphide, in lieu 'of lowering the melting point by oxidizing the metal melt to such a point that a certain amount of oxide of iron occurs in the slag, as described above. This, however, entails the risk of considerable losses in sulphur during the refining and in addition, even here it will be. necessary to oxidize and sulphurize the molten metal to such an extent that the slag will contain a considerable amount of iron in the case where it is desired to place practically all of the chromium into the slag. In carrying out my invention it is evident that a certain quantity of the original amount of chromium in the iron chromium alloy will be concentrated. in the slag through its conversion to oxide and sulphide, the amount carried over being controllable at will. If it be desired that practically all of the chromium be concentrated into the slag the result follows that at the same time an appreciable quantity of iron also is slagged, since a rather strong oxidation is necessary to completely oxidize the last traces of the chromium. If less chromium is slagged oif, then by the same token a smaller amount of iron will pass into the slag. If this dilution of the chromium-slag with iron becomes too great, a subsequent reduction with another chromium containing melt will be necessary even in this case.

A comparison between these two procedures, however, shows the advantage of the first described method to be as follows. The original content of chromium in the me a1 melt will be lowered in its first treatmefibwi h sulphide-oxide slag, bringing about a lesser volume of slag when the rest of chromium is slagged oil by oxidizing during the final refining, which is preferable if the Bessemer process is used for this refining. The possibility of obtaining a quantitative yield of the chromium in the slag is greater in this case;

and the concentration of chromium in the final sulphide-oxide slag, reduced by the second metal melt, will be very high.

In a second variation according to my invention it is possible to avoid the two-step process described in the foregoing and illustrated in the flow sheet, together with the required first and second refining furnaces. According to this variation the iron-chromium alloy is oxidized by means of ore 'in a. refining process, and at the same time the sulphurous material is added. Thereby the oxide and the sulphide of chromium are formed, and this slag, which is the chromiumrich product which is sought, is tapped from the furnace. but the drawback is present that the yield of chromium in the slag is less and the chromium content of the slag is lower than,is the case in the two-step process, described first.

The iron-content of the dilute refining slag obtained can be reduced with reducing agents other than chromium in a new melt. Thus carbon or metals not occurring to any appreciable extent 1n the initial melt. e. g. Al, Mn, may be used quite Thus no second refining is required,-

satisfactorily as reducing agents. The addition .of sulphurous material as described, in order to lower the melting point of the slag, is naturally made use of even in such cases.

.The sulphide oxide slag, rich in Cr, may then for instance be roasted-in air-withan eventual then subjected to a final refining, without any addition of sulphur, until practically all of the chromium is slagged off. This final refining can be made in a Bessemer converter, air serving as oxidizing means in this case, in an open-hearth furnace, or in an electric furnace. The slag obtained then will contain chromium oxide but also a rather high amount of iron oxide.

Due to the presence of this iron oxide the melting point of the slag will be rather low and it may be poured from the metal. In order to lower the Fe-content this slag is then reduced with a new charge of ferrochromium in a furnace with a neutral or a slightly reducing atmosphere. This step is equivalent to the above-mentioned prerefining. The Or of the ferrochromium and more or less of its C reduce the FeO, as well as melting point, the melting point of the slag would rise, rendering the' process more difficult. By means of adding S, for instance as FeS, chrom'i um sulphide is formed, together with chromium oxide, producing a more fusible slag. Thus a rather low melting point is maintained until most of the Fe et cetera has been reduced from the slag and replaced by Cr.

The process described in the two paragraphs immediately above starts with a ferrochrome alloy with for instance'a 45 per cent chromium content. The chromium is slagged off in two steps as illustrated in the flow sheet. In the first step, which takes place in the first refining furnace, part of the chromium is slagged off in a sulphideoxide slag; sufficient being removed, say, to reduce the chromium content from 45% to about 20%, whereupon the remaining 20 per cent chromium is rather completely slagged off in the second refining furnace in an oxide slag. This is shown in the flow sheet. This refining slag is then poured into the first refining furnace to react it with a new' iron chromium alloy together with added sulphide, and this cycle of operations 6 as previously described.

In consequence of equilibrium conditions of a lower'content of chromium than that of the sulphides and oxides of Cr and Fe in relation to the metals the slag will consist of a. preponderant amount of oxide and sulphide of chromium even when so much chromium has been slagged from the starting alloy, originally having a 45% Cr content, that only 10% Cr is left in the metal melt. By suitable choice of the. amounts of oxidizing and sulphurizing substances in the slag the percentage of residualchromium in the metal and consequently the ratio CrzFe in the slag can begiven-any value desired.

The slag rich in Cr is poured and is subjected to a further treatment, for instance roasting with subsequent reduction, in order to obtain Cr in a high-percentage Fe-alloy. Naturally, other methods used for extracting metal from ore containing sulphur may alsobe applicable.

The metal melt, which is pre-refined with the slag containing S, is then completely refined in the manner described above, producing a refining slag which is thereafter treated with another charge of iron chromium alloy, and so forth, provided that it is desired to concentrate all the chromium in the slag.

Naturally the final Cr-refining described can be omitted or restricted, depending upon whether it is desired to obtain an iron containing a certain, low percentage of Cr.

In addition to FeS, other metal sulphides having the sulphur less strongly chemically combined than in chromium-sulphide, as for instance NiS, CllzS, MoSz, can be used as sulphurizing substances, as a result of which the metalof the sulphide will be expelled fromthe slagand alloyed into the iron-melt, as described in the foregoing. v

To carry out the processes described hereinabove the furnaces utilized in the ordinary metallurgical refining processes may be used, such for example as mixers, converters, open-hearthand electric furnaces. Difierent combinations of furnaces may be used for carrying out the difierent partial processes. A basic lining is to be preferred. Thus, for instance, an open-hearth furnace may beused for the final refining of the metal melt; reduction of the refining slag obtained with the addition of sulphide may be brought about in'a mixer, in an open-hearth furnace, in an electric arc furnace or in an induction furnace.

I do not wish to be limited as to details of the process, as this may be. modified in many particular parts, withoutdeparting from the spirit.

alloy, by smelting the alloy'with ingredients containing oxygen and sulphur .less strongly chemically combined than in chromium oxide and chromium sulphide respectively, the oxygenand sulphur-chromium compounds being miscible and separating the slag rich in chromium from the iron containing metal melt.

2. The process of recovering, chromium from alloys containing iron andchromium by forming a, chromium-rich, low-fusing point slag, comprising concentrating chromium in a slag containing oxygen and sulphur while carrying over the iron to a subsequent metal melt which has a lower content of chromium than that of the alloy, by smelting the alloy with ingredients con- 5 taining oxygen and sulphur less strongly chemically combined than in chromium oxide and chromium sulphide respectively, the amounts of these ingredients being such that practically all of the chromium of the allfiy is concentrated in miscible form in the slag, and separating the" slag rich in chromium from the iron containing metal melt. v

3. In the process as claimed in claim 2, carryout the said smelting of the alloy with at least one oxide having the oxygen less strongly chemically combined than in chromium oxide and with at least one metallic sulphide having the sulphur less strongly chemically combined than in chromiumsulphide.

4. In the process as claimed in claim 2, carrying out the said smelting of the alloy with iron oxide and iron sulphide.

5. In the process as claimed in claim 2, carrying out the said smelting of the alloy with at least one oxide having the oxygen less strongly chemically combined than in chromium oxide and with sulphide of nickel. I

6.v In the process as claimed in claim 2, carrying out the said smelting of the alloy with at least one oxide having .the oxygen less strongly chemically combined than in chromium oxide andwith sulphide of copper.

7. The process of recovering chromium from alloys containing iron and chromium by forming a chromium-rich, low-fusing point slag, comprising concentrating chromium in a slag containingoxygen and sulphur while carrying over the iron to a subsequent metal meltwhich has a lower content of chromium than that of the alloy by smelting the alloy with ingredients con taining oxygen and sulphur less strongly chemically combined than in chromium oxide and chromium sulphide respectively, the amounts of these ingredients being such that practically all of the chromium of the alloy is concentrated in miscible form in the slag, reducing the amount of iron in the slag by adding a reducing agent, separating the slag rich in chromiumv from the iron-containing metal melt, and then recovering the chromium from the slag.

8. In the process as claimed in claim 7, carrying' out the said smelting of the alloy with at least one oxide having the oxygen less strongly chemically combined than in chromium oxide and 'with at least one metallic sulphide having the sulphur less strongly chemically combined than in chromium sulphide. I K

9. In the process as claimed in claim '7, carrying out the said smelting of the alloy with iron oxide andiron sulphide. l

10. In the process as claimed in claim7, carrying out 'the-said smelting of the alloy with at prising concentrating chromium in a slag containing oxygen and sulphur and while carrying over the iron to a subsequent metal melt which has a lower content of chromium than that of the alloy, by smelting the alloy with ingredients 5 containing oxygen and sulphur less strongly chemically combined than in chromium oxide and chromium sulphide respectively the oxygenand sulphur-chromium compounds being miscible, the amounts of these ingredients being such that a certain amount of chromium is left in the metal melt, and separating the slag rich in chromium from the iron containing metal melt.

13. The process of recovering chromium from alloys containing iron and chromium'by forming a chromium rich, low-fusing point slag, comprising concentrating chromium in a slag containing oxygen and sulphur and while carrying over the iron to a subsequent metal melt which has a lower content of chromium than that of the alloy, by smelting the alloy with ingredients containing oxygen and sulphur less strongly chemically combined than in chromium oxide and chromium sulphide respectively, the smelting being efiectuated in two stages in such a way that in the first stage the amounts of oxideand sulphide-yielding ingredients are such that a certain amount of chromium is left in the metal melt and in the second stage this metal melt is oxidized so far that its amount of chromium is oxidized rather completely, giving an oxide slag containing chromium and iron, and this slag being used as an oxide-yielding ingredient in the smelting of another charge of alloy with oxideand sulphide-yielding ingredients, the oxygenand sulphur-chromium compounds thus formed being miscible and separating the slag rich in chromium, obtained in the first stage, from the iron-containing metal melt.

14. In the process as claimed in claim 12, carrying out the said smelting of .the alloy with at least one oxide having the oxygen less strongly chemically combined than in chromium oxide, and with at least one metallic sulphide having the sulphur less strongly chemically combined than in chromium sulphide. 5

15. In the process as claimed in claim 12, carrying out the said smelting or the alloy with iron oxide and iron sulphide.

16. In the process as claimed in claim 12, carrying out the said smelting of the alloy with at 10 least one oxide having the oxygen less strongly chemically combined than in chromium oxide and with sulphide of nickel.

17.In the process as claimed in claim 12, carrying out the said smelting of the alloy with 15 at least one oxide, having the oxygen less strongly chemically combined than in chromium oxide and with sulphide of copper.

18. In the process as claimed in claim 13, carrying out the said smelting of the alloy with at 20 least one oxide having the oxygen less strongly chemically combined than in chromium oxide and with at least one metallic sulphide having the sulphur less strongly chemically combined than in chromium sulphide. 26

19. In the process as claimed in claim ,13, carrying out the said smelting of the alloy with iron oxide and iron sulphide.

20. In the process as claimed in claim 13, carrying out the said smelting of the alloy with at least one oxide having the oxygen less strongly chemically combined than in chromium oxide and with sulphide of nickel.

21. In the process as claimed in claim 13, carrying out the said smelting of the alloy with at least one oxide, having the oxygen less strongly chemically combined than in'chromium oxide, and with sulphide of copper.

KARL HELGE SIGFRID LoF ms'r. 0 

